Modulated carrier wave transmitters



Sept. 27, 1955 H. F. BARTLETT MODULATED CARRIER WAVE TRANSMITTERS FiledOct. 5, 1951 United States Patent MODULATED CARRIER WAVE TRANSMITTERSHugh Frederic Bartlett, Chelmsford, England, assignor to -MarconisWireless Telegraph Company Limited, London, England, a company of GreatBritain Application October 5, 1951, Serial No. 249,994

Claims priority, application Great Britain October 13, 195i) 3 Claims.Cl. 332-43 This invention relates to modulated carrier wave transmittersand more specifically to modulated carrier Wave transmitters employingso-called Class B modulation with a high frequency modulated stage inpush-pull. The ob ject of the present invention is to simplify andreduce the cost of such transmitters without detriment to theperformance thereof.

The invention is illustrated in the accompanying diagrammatic drawingsin which Fig. 1 shows a typical known circuit and Fig. 2 embodies thepresent invention.

In order that the invention may be the better understood the essentialsof a typical known push-pull Class B modulated high frequency stage willfirst be described, together with the defects which the presentinvention seeks to avoid.

This typical known circuit is shown in Fig. 1. In this circuit a Class Bmodulator which is represented by the valves V1, V2 has the anodes ofsaid valves connected to the opposite ends of the primary P of amodulation input transformer T, H. T. potential for the said anodesbeing applied from HTl through the mid-point M of this primary. Theaudio frequency currents for operating the modulation system are appliedto the primary winding shown at A of audio frequency input transformer Dhaving secondary winding E connected at opposite ends to the gridelectrodes of valves V1 and V2. The cathodes of these valves areearthed. A mid-tap connection in secondary winding E leads to the gridbias means, designated at GB, to the earth as shown. The transformersecondary S is earthed at one end through a suitable blocking condenserC7 and is connected at the other end through a low frequency choke LFCto a source of H. T. potential at I-ITZ. The said other end is alsoconnected through a low pass filter LPF to the two anodes of the valvesV3, V4 of the high frequency modulated stage, the connections of theseanodes containing the customary high frequency chokes HFCI, HFCZ, one ineach lead. The high frequency input to valves V3 and V4 is appliedthrough primary winding K of high frequency transformer L whosesecondary winding N is bridged by the two variable condensers C8 and C9connected in series and with the midcap 0 therebetween connected toearth. The opposite ends of secondary winding N connect throughcondensers C10 and C11 with the grid electrodes of valves V3 and V4,respectively. The grid bias system includes source G, B connectedthrough resistors R1 and R2 to the grid electrodes of valves V3 and V4as shown. Connected between these two valve anodes is a center tappedtuned circuit each end of which is connected to the appropriate anodethrough a blocking condenser C1 or C2. The tuned circuit usuallyconsists as shown of a center tapped tank coil TC in parallel with twosimilar, ganged tuning condensers C4, C in series, the point betweenthese condensers being connected to the center tap of the inductancethrough a further high frequency choke HFC3. The center point betweenthe two tuning condensers is directly connected to the two cathodes ofthe valves and earth. C6 designates a smoothing condenser. The output ofthe sys- "ice tem consists of tuned circuit W consisting of inductanceX, coupled with coil TC and tunning condenser Y in shunt with inductanceX, leading to the output circuit designated at terminals Z.

With this arrangement the largest part of the total ca pacity shuntedacross the modulation transformer is provided by the sum of thecapacities of the two blocking condensers C1, C2 and, if these be ofvalues also represented by C1 and C2 the shunt capacity is,approximately, and neglecting other smaller capacities (such as thoseprovided by the customary neutralizing condensers not shown) C1+C2. Inpractice these condensers must be comparatively large and expensive forthey have to offer very low impedance to the fundamental high frequencyand must be able to withstand the D. C. supply potential plus the radioand audio frequency potentialsan aggregate potential which, in a highpower transmitter, is large. Moreover, the provision of high capacity inshunt across the modulator is objectionable since, at the higher audiofrequencies, it makes the modulator load depart materially from aresistive character with consequent distortion and fall in response atthe upper frequencies. Indeed, it is due to the seriousness of thisdistorting effect that the low pass filter LPF is provided between themodulator and the high frequency stage and such a filter is, of course,costly.

The present invention seeks to avoid the foregoing defects and toprovide an improved arrangement wherein the total capacity in shuntacross the modulator is substantially reduced as compared to the knownarrangements and wherein, moreover, the blocking condensers (such as C1and C2 of Fig. 1) may be made smaller and cheaper and the need for a lowpass filter (LPF of Fig. 1) between modulator and high frequency stageeliminated.

According to this invention a push-pull modulated high frequency stagewith class B modulation and having a center-tapped tuned circuitconnected through blocking condensers to the anodes of the valves of thestage is characterized in that the center tap of the tuned circuit isconnected to the cathode point of the said valves through an additionalcondenser shunted by an impedance which is permeable to direct currentand of high value in relation to the reactance of said condenser atmodulation frequencies.

Fig. 2 shows a preferred embodiment of the invention.

In Fig. 2 the same references denote the same parts as in Fig. 1. Aswill be seen the difference (apart from the omission of the filter LPFbetween Figs 1 and 2 is that, in the latter circuit, instead ofproviding a direct connection between the two cathodes (and earth) ofthe valves V3, V4 and the center tap of the tuned circuit, an additionalcondensed C3 shunted by a leak resistance R is inserted in theconnection. With this modification, if C1 and C2 are, as before, thevalues of the blocking condensers and C3 is the value of the additionalcondenser designated by that reference, the shunt capacity across themodulation transformer is, approximately, (and neglecting the samecapacities as before) In a properly balanced push-pull circuit theadditional condenser C3 has to carry only the second harmonic of thefundamental high frequency and therefore its capacity may be made muchless than C1 and C2 so that the total shunt capacity becomes much lessthan C1+C2. This a substantial reduction of distortion due to themodulator load becoming reactive at high audio frequencies, so thatthere is no need for a low pass or second harmonic filter (such as LPFof Fig. 1) while the degree of linearizing negative feed-back which canbe employed is greatly increased. Also, since most of the audiofrequency potential is now across the additional condenser C3, the po- 3tential which the blocking condensers C1, C2 must be capable ofwithstanding is reduced and these condensers can be made physicallysmaller and cheaper. I claim:

1. A push-pull modulated high frequency stage with class B modulationcomprising a pair of valves each having at least a cathode, a controlgrid and an anode; a center tapped tuned circuit coil having one endconnected to one of said anodes through a blocking condenser and theother end connected to the other of said anodes through another blockingcondenser; a pair of ganged tuning condensers connected in series acrosssaid coil; a high frequency choke connected between the center tap onsaid coil and the junction point of said tunning condensers; and anadditional condenser shunted by a leak resistance connected between saidjunction point of said tuning condensers and the cathodes of the values;said leak resistance being of high value in relation to the reactance ofsaid additional condenser at modulation frequencies.

2. A modulated push-pull high frequency stage comprising a pair ofvalves each having at least a cathode, a control grid and an anode, acenter-tapped tuned high frequency circuit connected at its ends throughblocking condensers to the anodes of said valves, a circuit foreffecting class B modulation of said stage, said circuit including ananode potential supply path to the anodes of said valves and amodulation input impedance connected to a point in said anode potentialsupply path, said blocking condensers providing capacity effectivelyacross said impedance, a connection including an additional condenserproviding a capacitative path for high frequency between the center tapof said tuned circuit and the cathodes of said valves, and a directcurrent permeable impedance of high value in relation to the reactanceof said additional condenser at modulation frequencies in shunt acrosssaid additional condensers.

3. A modulated push-pull high frequency stage as set forth in claim 2wherein the cathodes of the stage are earthed and said direct currentpermeable impedance is a leak resistance.

References Cited in the file of this patent UNITED STATES PATENTS2,034,899 Ditcham Mar. 24, 1936 2,447,701 Hings Aug. 24, 1948 2,538,715Van Weel Jan. 16, 1951 FOREIGN PATENTS 457,568 Germany Apr. 27, 1929

